## Issue
I would send a message in the thread side pane and the main chat's
composer would then open its at-mention completion UI:
https://github.com/user-attachments/assets/25e79e05-8672-4f6e-b5cc-81a46638f229
## Summary
- Render inline composer autocomplete only for the focused rich-text
editor.
- Preserve editor focus when opening composer-owned mention controls.
- Add an end-to-end regression for sending in a side thread while the
main composer retains an agent mention.
## Root cause
The main channel composer and thread composer share the channel sending
state. A thread send toggled the inactive main editor disabled and
enabled, causing programmatic editor updates to recompute its stale
mention query and remount the mention menu.
## Verification
- Desktop unit suite: 5,556 passed
- Persistent agent audience E2E spec: 18 passed
- TypeScript typecheck
- Vite E2E build
- Biome and repository text/pubkey/file-size ratchets
---------
Signed-off-by: Matt Toohey <contact@matttoohey.com>
Co-authored-by: Jitter <2b2e6415e748c35180847a37aadae06a11e30dddc76b784e7fd9354c4eb42e7a@buzz.block.builderlab.xyz>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
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